I work in a bookstore. Everyone expect Christmas and Back to School to be busy, but there's another time that can be very stressful as well; World Book Day. It's in March, and every child in England and Ireland gets a voucher for a free book worth £1 or €1.50. A few years ago, we had a group of school children in on the day to meet an author and spend their vouchers. Chaos, but good chaos; the children are enjoying it, we're making a little bit of money on the books, it's all good, right?

Enter Phone Mommy. PM is, naturally, on her phone as she walks up to the counter, cutting off two children who'd been waiting to pay, and dumps an armload of things on the counter. I try to point out the children. She holds up one finger and talks on the phone. I try to serve around her. She shouts at me. Eventually another worker comes to deal with the children, and I start scanning PM through. Two of the books she's buying are on a Buy One, Get One; this is important.

So I finish scanning, I bag her belongings, and I announce the total. She scowls, puts a finger in her ear and keeps talking on the phone. I announce the total again. She turns her back on me. There's a queue of small children behind her, and when one tries to step past her to pay - which I would have allowed, since she didn't seem to be in any hurry - she finally hangs up and looks back at me.

I announce her total for the third time. She throws down three vouchers. Now, they're supposed to be one per transaction, but we're allowed to waive that if we think it's necessary and I want this woman gone. However, while she is buying three applicable books, two of them are the BOGOF and so she's only paying for two books. I apply two vouchers and tell her the new total.

"No, it should be cheaper."

I explain that since she's only paying for two of the books, she can only use two vouchers.

"But I have three vouchers."

Yes, but one of your books is free, and I can't give you money off a total of nothing.

"But it should be cheaper!"

Wash, rinse, repeat. After about ten minutes - and my poor coworker is dealing steadily with small children the whole time - PM finally gathers up her belongings and leaves, still grumbling "But I have three vouchers!" I carefully hide what I'd really like to say and go back to serving touchingly grateful children.